In the chemical pulping of fibrous, cellulosic material for producing paper and board, the raw material is treated with chemicals, for example, sodium and sulfur compounds, at elevated temperature. Typically, this treatment is performed at super-atmospheric pressure to ensure the aqueous solutions remain in liquid form. The chemicals react with the organic and non-organic constituents of the raw material such that some of the organic and non-organic constituents are dissolved to yield a product consisting of cellulose fibers in an aqueous slurry of dissolved reaction products. The slurry is typically cleaned and dewatered to provide an essentially pure form of cellulose fibers for paper making.
Prior to formally cooking the comminuted cellulosic fibrous material at elevated temperature, for example, between 140-180.degree. C., the comminuted cellulosic fibrous material, typically hardwood or softwood chips (though many other types of comminuted cellulosic fibrous material are used), is pretreated (e.g. steamed)to remove entrained air, impregnate the material with cooking liquor, and initiate the heating process, among other things. Though historically this pretreatment raised the temperature of the cellulose material as quickly as feasible or allowed by the feeding equipment, it has now been discovered that the rate of heating need not be rapid, but a more gradual heating or cooler treatment is preferred. Such treatment produces a cellulose pulp having improved fiber properties, for example, increased fiber strength.
Several copending applications disclose methods of effecting this cool pretreatment. For example, U.S. Pat. No. 5,674,359 the disclosure of which is incorporated by reference herein! discloses a two-step cool impregnation process. In the first step the chips are treated with alkali at a first concentration at a temperature between 80 and 110.degree. C. In a second step, the chips are treated with a second alkali concentration, higher than the first alkali concentration, at a temperature between 110-150.degree. C. The slurry of chips and liquor is then cooked at a temperature of between 150-180.degree. C. Copending application Ser. No. 08/460,723 filed on Jun. 2,1995 the disclosure of which is incorporated by reference herein! also discloses a method of treating pulp at a cool temperature. This application discloses a process of treating chips by supplying a cool source of alkali during impregnation to neutralize the cellulose-damaging wood acids. In addition, copending application Ser. No. 08/729,022 filed on Oct. 10, 1996 now U.S. Pat. No. 5,736,006, the disclosure of which is incorporated by reference herein! discloses a method of treating and feeding a slurry of comminuted cellulosic fibrous material to a digester at a relatively cool temperature. This application uses the novel chip bin disclosed in U.S. Pat. Nos. 5,500,083; 5,617,975; and 5,628,873, marketed under the name DIAMONDBACK.RTM. by Ahistrom Machinery Inc. of Glens Falls, N.Y., and the novel feeding system disclosed in U.S. Pat. Nos. 5,476,572; 5,622,598; and 5,635,025 (the disclosures of which are hereby incorporated by reference herein), which is marketed under the trademark LO-LEVEL.RTM. by Ahistrom Machinery.
However, none of these treatments disclose the most effective means of cool pretreatment and then heating to cooking temperature as disclosed by the present invention.
U.S. Pat. Nos. 5,489,363; 5,536,366; 5,547,012; 5,575,890; and pending application Ser. No. 08/484,315, filed on Jun. 7, 1995 the disclosure of all of which are incorporated by reference herein! disclose a novel cooking method and equipment which is marketed under the name LO-SOLIDS.RTM. by Ahlstrom Machinery. This process includes the control and minimization of dissolved organic material throughout the cooking process in order to improve, among other things, pulp strength, bleachability, and operability of the cooking process.
The present invention combines and supplements the unique features of the processes described above to provide a pretreatment and cooking process that yields improved fiber properties, for example, improved strength, improved distribution of heat and chemical which can result in improved pulp uniformity, and improved operability of the cooking process, while minimizing the use of energy and cooking chemicals. One embodiment of the invention comprises or consists of a method for continuously cooking cellulosic fibrous material, comprising the steps of continuously and sequentially: (a) treating the cellulose fibrous material in a first stage (e.g. zone) with an alkali-containing liquid at a temperature of between about 60-120.degree. C. (preferably 95-105.degree. C., e.g. below 100.degree. C.) and at a first alkali concentration over 10 g/l (e.g. about 20-35, or 25-35,g/1, as NaOH); and (b) treating the cellulose fibrous material in a second stage (zone) with a liquid containing a second alkali concentration at least 5 g./l less (e. g. about 7-15 g/l less) than the first alkali concentration (e.g. between about 10-20 g/l, as NaOH) at a temperature of between about 120-160.degree. C. (e.g. about 130-150.degree. C.) ; and (c) cooking the cellulose material from step b) at a temperature of between about 140-180.degree. C. (e.g. about 150-170.degree. C.) to produce a chemical cellulose pulp (e. g. kraft pulp). The invention may also include an intermediate step (d), between steps (a) and (b), of removing some of the alkali-containing liquid from the material (and preferably at some point replacing it with liquid having a significantly lower dissolved organic material concentration than the removed liquor, e.g. by adding filtrate or water). The treatment step (b) may be performed counter-currently.
The method of the invention also may include other details or modifications. For example, step (b) may practiced using white liquor, and step (a) is practiced using white, green, or black liquor (including mixtures thereof). Step (c) may be practiced at an alkali concentration of between about 18-40 g/l (e. g. 18-25) throughout, and there may be the further step of washing the material after step (c).
Steps (d), (b) and (c) are typically practiced in an upright continuous digester, and step (a) may practiced, but only in part, in an upright continuous digester (the rest of the cool impregnation taking place in the feed system). Of course, more than one vessel may also be used, e.g. an impregnation vessel and a digester.
Step (b) may be practiced by withdrawing liquid from the digester, heating the withdrawn liquid and recirculating the withdrawn liquid back to the digester, adding filtrate or water to the withdrawn liquid prior to heating and recirculation, in sufficient amount to approximately make up for the liquid extracted in step (d), and adding white liquor to the recirculated liquid in an amount sufficient to insure the desired alkali concentration thereof.
The invention also relates to a kraft pulp made by practicing the method described above, and having enhanced fiber properties, including increased fiber strength, compared to kraft pulp made by a process using conventional temperatures and alkali concentrations in the practice of steps (a) and (b).
According to another aspect of the present invention, a method of treating a slurry of comminuted cellulosic fibrous material using a feed system to an upright continuous digester, the digester having first and second screen assemblies disposed in the digester near the top thereof, the second screen assembly below and spaced from the first screen! is provided. The method comprises the steps of continuously and sequentially: (a) at least partially in the feed system subjecting the comminuted cellulosic fibrous material to cool impregnation with an alkali-containing liquid having a first alkali concentration of at least 10 g/l expressed as NaOH, and at a temperature of between about 60-120.degree. C. (e.g. 80-110.degree.), and feeding the cool impregnated-material to the top of the digester; (b) extracting some of the alkali-containing liquid from the material using the first screen assembly; (c) between the first and second screen assemblies, treating the material with a second liquid having a second alkali concentration at least 5 g/l less than the first concentration, and at a temperature of 120-160.degree. C.; and (d) below the second screen assembly cooking the material at a temperature of between about 140-180.degree. C. The details of the temperatures and alkali concentrations preferably are as set forth above with respect to the first aspect of the invention.
The method also typically comprises the further step (e) of withdrawing some liquid from the slurry using a third screen assembly, above the first screen assembly, and recirculating the liquid withdrawn in step (e) to the feed system. The feed system used typically includes a chip bin connected to a chip tube, in turn connected to a helical screw pump, in turn directly connected to a high pressure feeder for feeding the slurry to the top of the digester. Also, step (c) is typically practiced in part by the substeps of (1) withdrawing liquid from the slurry using the second screen assembly, (2) recirculating at least the majority of the withdrawn liquid to the interior of the digester at about the level of the second screen assembly, (3) adding cooking liquor and make up liquor to the recirculated liquid to produce an augmented liquor, and (4) heating the augmented liquid, substeps (1)-(4) being practiced so that the liquid recirculated to the digester has a temperature of between about 140-160.degree. C., and an alkali concentration of between about 10-20 g/l expressed as NaOH. Substep (3) is preferably practiced by adding filtrate or water as make-up liquid, and white liquor as cooking liquor.
It is the primary object of the present invention to provide a method of producing chemical pulp with optimum fiber properties from wood chips or the like by using cool impregnation and desirable alkali concentrations and temperatures at significant times during treatment of the chips. This and other objects of the invention will become clear from an inspection of the detailed description of the drawings, and from the appended claims.